The invention relates to a cylinder liner to be inserted into the cylinder bore of an engine block. The liner is made in a free-form method by thermal spraying of a spray material onto a mandrel serving as a mold body to obtain a cylinder wall.
The engine block of an internal-combustion engine typically comprises a cast-iron or aluminum alloy. Cylinder liners are cast in the engine block which is made by a casting method. Wear-resistant metals or metal alloys serve as the material for the cylinder liner which may be manufactured in numerous ways.
A method of producing aluminum cylinders having cast steel liners is disclosed in German Patent No. 12 82 243. First, a steel tube is made by thermal spraying, then inserted, as a separate body, into the metal block. To connect the liner to the cast material, a vacuum is generated in the liner to cause the melted aluminum to penetrate the pores of the steel tube. Multi-layer cylinder liners have been developed to simplify this complex connecting technique. The layer forming the running surface comprises a wear-resistant and scorch-mark-resistant material. An outer cover layer comprising, for example, an aluminum alloy is sprayed onto the layer forming the running surface.
In the casting process for liners of the above-outlined type the liquid engine-block material fuses with the cover layer. German Patent No. 196 05 946, to which corresponds U.S. Pat. No. 5,929,405, discloses such a cylinder liner and a method of producing the same.
It has been found that in complex geometrical configurations of the engine block the hardening process, in conjunction with the shrinkage of the cast material, leads to stresses in the cooled cast piece. This results in considerable distortions in the region of the liner,
Because the cylinder liners often require machining after casting, the wall thicknesses of the liners may be partially reduced. During engine operation, these regions represent critical zones in terms of thermal and mechanical stability.